In the fabrication of microchips for use in the electronics and computer industries, a wafer is typically fabricated which comprises a plurality of individual dies (or chips) commonly arranged in a grid pattern. The sections of a wafer between the individual dies are termed streets. FIGS. 1A and 1B illustrate an exemplary wafer 10. FIG. 1A is a plan view of a wafer, while FIG. 1B is an enlarged view of section A in FIG. 1A. Reference numerals 14 denote individual dies and reference numerals 12 denote the streets separating the individual dies 14. Streets 12 are simply areas of the wafer where no componentry has been placed and which define the boundaries of each individual die 14. The integrated circuitry and other componentry appears on only one surface, e.g., surface 15, of the wafer. The opposing surface (not shown in FIG. 1) is blank.
The individual dies 14 comprising a wafer are removed from the wafer by sawing through the wafer along all of the streets, thus physically separating the wafer in both axes into the individual dies.
Standard industry practice for separating each wafer up into individual dies is described below.
First, the wafer 10 is placed upside down (i.e., with the circuit side 15 of the wafer 14 facing downwardly and the non-circuit circuit side facing upwardly) on a flat surface. A metal film frame defining an opening is laid over the wafer with the wafer being within the perimeter of the opening in the film frame. A plastic film is then laid over the metal film frame and back (non-circuit side) of the wafer. Preferably, the plastic film is coated with adhesive on the side which contacts the film frame and the back of the wafer. Force is then applied between the film and the film frame to cause the film to adhere well to the frame. One possible technique for applying the force is to pass a rolling pin over the wafer and film frame to adhere the film to the back side of the wafer and to the surface of the film frame. The wafer is now mounted to the film which, in turn, is mounted to the film frame.
The wafer, film, and film frame combination (hereinafter the film frame assembly) is then turned over so that the circuitry on the top of the wafer now faces upwards. The film frame assembly is then placed on a movable pallet in a sawing station. The sawing machine typically comprises a camera and a computerized optical system utilizing optical pattern recognition software which controls movement of the pallet so as to align the streets on the wafer with the saw blade. This can also be done manually by observing a video image obtained by the camera on a screen and manually adjusting the position of the pallet to the desired location. The pallet and wafer is then advanced under the saw blade to cut through the street. Commonly, a wafer has a first plurality of parallel streets aligned in a first direction and a second plurality of parallel streets are aligned orthogonal to the first plurality of streets thus defining a grid with individual dies comprising the blocks between the orthogonal streets.
Accordingly, the wafer will be advanced through the saw blade to cut along a street, shifted laterally to the cutting direction a distance equal to the spacing between the parallel streets and advanced through the saw blade to cut the next street. This process is repeated until all of the first plurality of parallel streets are cut. The pallet and wafer are then rotated 90.degree. and the wafer is advanced through the blade a number of times again to cut through all of the parallel streets in the second orthogonal direction.
The saw blade height is adjusted such that it will cut completely through the wafer and contact and score, but not cut through, the film. The plastic film may be a "Mylar" film of approximately 3 mils in thickness. The blade height would be set, for instance, to cut 1.5 mils into the film.
During the sawing process, water is jet sprayed over the surface of the wafer as well as over the surface of the saw blade to cool the wafer and saw blade.
After the sawing operation, the wafer is transported to a cleaning station where it is sprayed with de-ionized water and brushed to clear away any remaining silicon slurry created by the sawing operation. The wafer typically is then dried after the water flow and brushing operations are completed. The drying may be accomplished in the cleaning station by rapid rotation of the wafer or, alternately, the wafer may be removed to an oven for heat drying. Other drying options are also available.
After cleaning, the film frame assembly is transported to a pick-and-place station where the now detached individual dies are to be removed from the film (to which the dies are still adhered).
The pick-and-place station removes the individual dies from the film and places them, for instance, in a carrier. Commonly, the metal film frame (to which the individual dies are still adhered) is slid into a movable slotted holder in the pick-and-place station which is located above an anvil comprising a needle or needle cluster. A camera is positioned above the anvil and the film frame assembly to obtain an image of a die on the film frame assembly. The image is processed in an optical pattern recognition system and the position of the film frame assembly is adjusted to line up a die with the anvil. The film frame assembly is then clamped in place and a mechanism grasps the film beyond the perimeter of the wafer and stretches the film radially outward. The stretching of the film serves to reduce the film adhesion to the dies at the edge of the dies. After the stretching operation, the anvil is used to further separate the dies from the film. The anvil contains a needle or needle cluster which is advanced upwardly to contact the film underneath the selected die, pierce the film and push the die upwards.
Also under control of the computer and pattern recognition software, an arm having a vacuum-equipped probe is positioned over the top surface of the die. The arm lowers the probe into contact with the die and the vacuum pressure causes the die to attach to the probe. The arm is then controlled to lift the die up and away from the film and transports it over to a grid carrier where the arm descends to position the die in a slot in the carrier and the vacuum is turned off so the die is placed in the carrier. Typically the pick-and-place station will comprise a second camera positioned to obtain an image of the grid carrier and computer control for assuring that the dies are placed in the appropriate receptacles in the grid carrier. The die can then be delivered to the next station for further processing.
U.S. Pat. application Ser. Nos. 07/569,080, now abandoned, 07/872,037, now U.S. Pat. No. 5,314,572 and 07/899,765, now abandoned, pertain to a monolithic accelerometer microchip. Those applications are assigned to the same assignee as the present application and their disclosures are incorporated herein by reference. The microchip comprises both a suspended microstructure for detecting accelerative forces and integrated circuitry for resolving the signal from the sensor into a useful output. The sensor is a variable capacitance capacitor, the capacitance of which changes responsive to acceleration as explained below. One node of the capacitor comprises a polysilicon bridge suspended above the substrate on a series of posts. The polysilicon bridge comprises a suspended longitudinal beam having a plurality of fingers (hereinafter beam fingers) extending transversely therefrom. For each beam finger, there is a corresponding stationary finger positioned parallel and in close proximity thereto. The stationary fingers comprise the other node of the capacitor. The bridge and all of the fingers are electrically conductive. The bridge, including the beam fingers, is charged to a different voltage than the stationary fingers. The polysilicon is resilient such that the bridge, comprising the fingers, will sway under accelerative force such that the spacing between the beam fingers and the stationary fingers, and thus the capacitance of the sensor, will change. The capacitance signal from the sensor is fed to the resolving circuitry on the same substrate which creates a useful output signal indicative of the magnitude of the accelerative force.
When the monolithic accelerometer chip is fabricated, the circuitry portion of the chip is coated with passivation to protect it. However, the microstructure cannot be passivated since it must be able to move freely. In the preferred embodiment, the microstructure is positioned essentially in the center of the microchip (i.e., the die). Due to the fact that the microstructure is comprised of extremely small sections of polysilicon so that it is resilient and the fact that it is not coated with passivation, the microstructure is extremely fragile and great care must be taken during fabrication, up to and including the final packaging steps, not to harm the microstructure. If a wafer comprising a set of monolithic accelerometer dies was passed through the standard die separation process as described above, the microstructures would be destroyed. The water jet sprays used in a sawing process would destroy the microstructure. If any microstructures happen to survive the water spray during the sawing operation, they would be destroyed during the subsequent spraying and brushing in the cleaning operation. Further, if any microstructures survived those two steps, they would probably be destroyed in the pick-and-place station by the vacuum-equipped arm which picks up the dies and places them in the grid carrier.
Accordingly, it is an object of the present invention to provide an improved method and apparatus for separating the individual dies from a wafer containing a plurality of dies.
It is a further object of the present invention to provide a method and apparatus for separating individual dies from a wafer comprising a plurality of dies which will not damage fragile portions of the dies.